Field of the Invention
The invention relates to a method for determining the distance between periodic structures on an integrated circuit or a photomask in which a map of the integrated circuit or the photomask is recorded.
The mean distance between periodic structures on semiconductor wafers or photomasks is determined by recording measured values for the edge position at a number of measurement points along a structure edge, and by using the measured values to determine the distance between periodic structures on the semiconductor wafer or the photomask.
U.S. Pat. No. 6,020,957 discloses a system and a method for investigating semiconductor wafers in which a number of areas on a semiconductor wafer can be investigated at the same time. In this case, a monochromatic light source is used to illuminate the semiconductor wafer surface. An optical dark-field system gathers the light reflected from the surface of the semiconductor wafer and filters out patterns which correspond to correct periodic wafer structures. In the process, filtering is carried out using the Fourier method. The filtered light is further-processed by a digital signal processor. Defective wafer surfaces are identified by image comparison. However, the known method does not describe any method for determining the distance between periodic structures using Fourier transformation.
A method and an apparatus for recording structure features on a substrate surface are known from Published German Patent Application DE 10 006 782 A1. The method uses the following steps: raising a number of structure features from the rest of the substrate surface, optically recording one surface area of the substrate surface in which at least a number of the raised structure features are located, carrying out the optical recording digitally or in analog form with subsequent digitization, determining at least one mathematical function as a function of the brightness or color steps which alternate regularly on the substrate surface, and determining the amplitude spectrum of that function. The amplitude spectrum of that function is obtained by Fourier analysis.
It is accordingly an object of the invention to provide a method for determining the distance between periodic structures on integrated circuits or photomasks.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for determining the distances between periodic structures on an integrated circuit or on a photomask, that includes steps of: recording a map selected from the group consisting of a map of an integrated circuit and a map of a photomask; calculating a Fourier Transform of the map; determining a first-order maximum and a zero-order maximum in the Fourier transform; and calculating a distance between the periodic structures on the integrated circuit or the mask from a distance between the zero-order maximum and the first-order maximum.
In accordance with an added feature of the invention, the method includes: calculating a power spectrum from the Fourier transform; and performing the step of calculating the distance between the periodic structures using a distance between a first-order maximum and a zero-order maximum in the power spectrum.
In accordance with an additional feature of the invention, the method includes evaluating the power spectrum to determine an orientation of one of the periodic structures.
In accordance with another feature of the invention, the method includes: integrating the power spectrum along different straight lines that run through the zero-order maximum; and identifying one of the straight lines, having an integral that is greater than that of all others of the straight lines, as an orientation axis; the one of the periodic structures being configured at an angle of 90xc2x0 with respect to the orientation axis.
In accordance with a further feature of the invention, the method includes: producing the map in a form selected from the group consisting of an optical image and a digital image.
One major step in the invention is for a map of the periodic structure to be recorded, and to be subjected to Fourier transformation. The distance between the periodic structures is determined from the position of the first-order maximum in the Fourier spectrum. This provides a simple way for obtaining and evaluating overall information relating to the recorded structure.
A power spectrum is advantageously calculated utilizing the Fourier transformation, and the distance between the periodic structures is calculated from the distance between the first-order maximum and the zero-order maximum of the power spectrum. The use of the power spectrum offers a simple and reliable method, by means of which it is possible to determine the distances between the periodic structures precisely.
It is also advantageous to determine the orientation of the periodic structure by integration of the power spectrum.
The orientation of the periodic structure is preferably obtained by integrating the power spectrum along different straight lines that run through the zero-order maximum and by selecting the straight line whose integral has the greatest value as the orientation axis. This orientation axis is configured at an angle of 90xc2x0 with respect to the periodic structure. The orientation of the periodic structure can be determined precisely by means of the described method.
Depending on the embodiment, the map of the periodic structure can be produced in the form of an optical image or in the form of a digital image, and can be evaluated in an appropriate manner.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for determining the distance between periodic structures on an integrated circuit or a photomask, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.